1 You may think it's easy to tell the difference between plants and animals. You might say that plants grow on the ground and don't move, but animals can move freely when they want. What about those plants that float down streams or with ocean currents? On the other hand, there are some animals, like sponges, that are stationary. Sponges live in all oceans. They attach themselves to the surface of the rocks and do not show any movement.2 You might also say that plants are green and animals are not. But if you think a little harder, you'll remember many animals that are definitely green --some fish, birds, insects, and snakes.3 So, what makes plants and animals different? For many years scientists have tried to answer that question. They came up with several rules to distinguish plants from animals. But as scientists learn more about nature, they find exceptions to the rules.